tube choices for an ST70

So a little over a year ago I got an ST-70, and replaced all caps and resistors. I got the WC tube kit from tubes and more, and with maybe the 40 hours or so of running, the tubes on one channel would keep changing bias. I was constantly adjusting it. The problem followed the tubes when I would swap them to the other channel. Well listening to it one day, the channel with the tubes got a load hum, and by the time I got to it, the plates were cherry red, and the rectifier arced and died. So, did I just get a bad batch of tubes? Or should I try a different brand? I have 3 pairs of matched 6L6GC's could I use them? Maybe KT88's or similar? I'm really looking for ideas and past experiences.

Thanks,
Jeremy